1. Field of the Invention
The invention relates to a stabilizer system for use as a light-protection agent for polyolefins.
2. Description of the Background
Synthetic thermoplastic high polymers, particularly polyolefins, are known to suffer degradation of their optical and mechanical properties when subjected to light, heat, or atmospheric oxygen. The degradation leads to appreciable cracking, chalking, discoloration, brittleness, and other undesirable effects within the polymer. In order to improve the resistance of the polymers to these degrative influences, stabilizers are added to the polymers in order to counteract these degrative influences. Invariably antioxidants are added to polyolefins so that they are capable of resisting oxidative attack during processing and subsequent use. Beyond this basic stabilization measure, it is customary to add light-stabilizers to the polyolefins in order to extend service life, particularly in outdoor use, because the antioxidants are only slightly effective in suppressing photooxidation caused by the UV component of light. Accordingly, UV absorbers of the benzotriazole or benzophenone type, and/or nickel quenchers, are commonly used for light-stabilization of polyolefins. These stabilizers enable substantial improvement of the light stability of the polyolefins; however, they have some disadvantages as well. Thus, UV absorbers do not provide the polymer surface with adequate protection, since their absorption increases exponentially with the concentration of the agent and the distance from the surface, according to the Beer-Lambert law. The consequence, particularly for thin layers (films, fibers, etc.), is premature surface cracking. Nickel quenchers work well in thin layers, but they are themselves colored (as are the benzophenones) which limits their area of applicability.
The light stability of polyolefins can be increased markedly if derivatives of polyalkylpiperidines are used as light-stabilizers. These substances belong to a class of sterically hindered amines. They are regarded as the most effective light-stabilizers for polyolefins. The preferred such agent for polyolefins is the 2,2,6,6-tetramethyl-4-piperidinol diester of sebacic acid, which is marketed, e.g., by Ciba-Geigy under the trade name TINUVIN.RTM. 770, and commands the greatest market share among the sterically hindered amines.
Esters of polyalkyl-4-piperidinols with mono-, di-, and polycarboxylic acids have long been known, and are described, e.g., in Ger. OS 19 29 928. Synthetic polymers such as polyolefins, polyvinyl chloride, polyvinylidene chloride, polyurethanes, and polyamides, are stabilized against photo- and thermal degradation if they contain, among other components, esters of the formula: ##STR2## wherein, in the case of an ester of an aliphatic monocarboxylic acid, n=1, and R.sub.3 is an aliphatic monoacyl group, possibly containing phenyl-group or phenoxy-group substitutions, and having 2 to 18 C atoms. In the case of an ester of an aliphatic dicarboxylic acid, n=2, and R.sub.3 is an aliphatic diacyl group. Tabular evaluation of the compounds indicates that the dicarboxylic acid diesters of 2,2,6,6-tetramethyl-4-piperidinol are superior to the monocarboxylic acid monoester counterparts, in light-stabilizing action.
Subsequently, a large number of patent and other publications have been concerned with further modification of the ester molecule (see Ger. OSs 22 04 659, 22 58 725, 26 21 870, and 32 33 943) or the addition of other structural elements to the 2,2,6,6-tetraalkylpiperidine.
Also, frequently descriptions are found of the use of mixtures of these compounds with other additives, for light stabilization, whereby synergistic, additive, or antagonistic effects can occur, depending on the structure, stabilizing mechanisms, and the intermediate products occurring in the event of photolysis. Synergistic effects between TINUVIN 770.RTM. and benzotriazole absorbers in polypropylene plates have been reported by Gugumus, 1979, in Scott., G., Ed., "Developments in Polymer Stabilization", pub. Appl. Science Publishers, London, pp. 280-308. However, according to Allen et al., 1980, Polymer Degrad. Stab., 2, 129 ff.; 1981 ibid., 3, 199, no synergism can be observed between TINUVIN 770.RTM. and benzophenones or benzotriazoles as additives in polypropylene films. According to the data from a study of the interaction between sterically hindered piperidines and UV-absorbers with regard to light-stabilization of polybutadiene (Lucki, 1985, Polymer. Photochem., 6, 273-291), there is antagonism between UV-absorbers of the benzophenone type and sterically hindered piperidines. Also, no synergism with benzotriazole absorbers was detected.
Thus in the publication "Technische Information TINUVIN 770.RTM." [("Technical bulletin TINUVIN 770.RTM.")] of the firm Ciba-Geigy, dated 1975, 1977, and 1980, the diagrams for light stability for the combination of the 2,2,6,6-tetramethyl-4-piperidinol diester of sebacic acid with benzotriazole absorbers in polyethylene films show no synergism, and the diagrams for the same combination in polypropylene in thin or thick layers also show no synergism or only slight synergism. Finally it is only noted that the use of UV absorbers as co-stabilizers in connection with thick layers may be of advantage. On the otherhand, in studies with ABS polymer as a substrate, a substantial synergism of the stabilizer mixture was detected.
In the likes of, Ger. OSs 24 17 535 and 33 43 676, and Eur. Patents No. 0 002 753, 0 154 071, and 0 180 992, increased or synergistic light-protective effects are described for mixtures of sterically hindered piperidines with benzotriazoles in connection with PVC, ABS, PUR, polyamides, crosslinked polyolefin foams, and metal-pigment-containing acrylate and polyester paints. These stability tests were in all cases more successful with sterically hindered piperidines of the dicarboxylate ester or polycarboxylate ester type or with polymeric esters. Nothing is known about the behavior of aliphatic monocarboxylate esters in this connection.
The evaluation criteria described in the literature for tests of light-stabilizing action nearly exclusively involve measurements which relate to the mechanical properties of a polymer, e.g. residual ultimate tensile strength, residual elongation, bendability, reduction of melt viscosity, carbonyl index, and the like. Only with regard to stabilization of paints and varnishes are additional criteria such as surface gloss and cracking evaluated. Thus, quite irrationally, the most important aspect of the light-stabilizing action of sterically hindered amines, namely their outstanding surface protection, is not taken into account in testing polymers. An optically unblemished surface quality is an important criterion in evaluating the useful characteristics of a polymeric material. This objective is not only important from decorative considerations. It turns out in practice that when a combination of sterically hindered amines with UV-absorbers is employed, the ultimate tensile strength, bendability, and the like are no longer the decisive criteria if there is early onset of surface crack formation. That is, the increasing damage to the surface clearly reduces the penetration of harmful radiation, much of which radiation is now scattered or absorbed before penetrating, and as a result the mechanical properties are undegraded for a longer time than is the case with a system in which surface crack formation starts much later. However, despite satisfactory mechanical properties, the usefulness of the material is reduced. The tendency toward relatively early surface crack formation is shown by sterically hindered amines of the dicarboxylate ester type, such as TINUVIN 770.RTM.. A need therefore continues to exist for an improved stabilizer system for the stabilization of polyolefins.